1. Field of the Invention
The present invention relates generally to the field of gel-filled streamer cables and in particular to placing a pressure-sensitive hydrophone inside of a gel-filled streamer section to isolate the hydrophone from shear waves and mechanical forces.
2. Summary of the Related Art
Marine seismic exploration and data acquisition is often conducted by a host vessel towing a seismic streamer at a given depth through the ocean or some other body of water. The seismic streamer is provided with a plurality of acoustic sensitive transducers disposed at appropriate intervals along its length. An acoustic source providing acoustic wave energy is provided in the vicinity of the seismic hydrophone cable or streamer, by an air gun or other suitable means. The acoustic wave energy travels downward through the earth with a portion of the acoustic energy being reflected upward at earth formation levels where there is a contrast in the velocity propagation characteristics of the strata.
The magnitude of the reflected signals is extremely small, thus making it essential to minimize extraneous noise. One source of such noise is the mechanical longitudinal waves, which are propagated along the strength member of a streamer. The unsteady movement of the seismic vessel towing the streamer as the vessel heaves, pitches and rolls in an unsteady ocean environment generates the mechanical longitudinal waves. This unsteady motion is transmitted to the streamer via its attachment to the vessel may also result in lateral, shear, Rayleigh and torsional waves depending on the structure of the cable. The seismic transducers detect these various undesirable waves, thereby reducing the signal to noise ratio of the desired pressure wave seismic signals reflected from subterranean formations. Therefore, it is an object of the present invention to provide a marine seismic cable in which the seismic transducers are isolated from shear waves and from mechanical waves generated in the cable by surface waves and the non-uniform movement of the seismic vessel towing the cable.
To perform seismic surveys over water-covered areas, such as offshore, or to obtain information regarding subterranean Earth formations for recovery of hydrocarbons (oil and gas), one or more streamers of hydrophones are towed behind a vessel designed for performing seismic surveys. For three-dimensional seismic surveys, several streamers (generally between 4 and 12) are deployed simultaneously, each such streamer extending usually between three (3) and eight (8) kilometers. Each streamer is normally formed by serially joining shorter streamer sections of 75 meters to 150 meters in length, referred to in the art as “active streamer sections.” The streamer is generally deflected downward by a paravane and towed twelve to thirty feet below the water surface to reduce the effects of surface waves and surface reflected noise on the hydrophones. However, towing streamers at such depths requires greater pulling force, hence creating a need for larger and more powerful towing vessels, which in turn increases operating costs.
Typically, each active streamer section of commercially available streamer is made up of a flexible sealed tubular outer jacket manufactured from polyurethane or a similar material. Multiple strength members, generally between two and five in the form of cables made of steel or other high strength materials, such as those sold under the trade names of Kevlar or Vectran, are spaced apart radially around the longitudinal axis of the cable and run along the entire length of the active cable section. Typically, the strength members are deployed near the inside surface of the flexible tubular member to absorb the pulling forces when the streamer is towed behind the vessel. Hydrophones are typically placed in the center space between the radially spaced strength members. To detect very small reflections from the subterranean formations, groups of hydrophones equally spaced along the longitudinal axis of an active streamer section (typically in groups of between 8 and 14) are placed in each active streamer section. A one hundred (100) meter active streamer section typically provides between 96 and 150 hydrophones.
Typically, electronic circuitry, such as preamplifiers along with connecting wires, are placed between the active streamer sections to digitize analog hydrophone signals and provide two-way signal and data communication between the active streamer sections and control units located on the towing vessel. Since the streamer cables are to be towed at a predetermined depth below the water surface, the cable is deployed having predetermined buoyancy on deflected upward and downward to a desired depth by a paravane attached to the streamer cable. In liquid-filled cables, buoyancy is adjusted by filling all of the empty space inside the outer housing with a non-conductive buoyant fluid, such as kerosene.
Fluid-filled streamer cables suffer from a number of significant problems. The outer jacket is typically only a few millimeters thick and thus, is, easily penetrated by shark bites or other physical hazards encountered during towing, storage and deployment. Moreover, fluid-filled streamer cables are normally spooled onto large drums for storage on the vessels and often rupture during winding (spooling) and unwinding operations. Additionally, the outer jacket can be easily ruptured during towing when fishing boats inadvertently pass over the streamer and damage the streamer jacket on contact. Fish bites or streamer entanglement with offshore structures can also rupture the outer jacket. Seismic survey companies spend large amounts of money in repairing such cables and are typically forced to keep excessive inventory of such cables as spares for damaged cables. Outer jacket ruptures during surveying operations can require shut down of the surveying operations. Such down time can be very expensive due to the large capital cost of the vessels and the lost time of the crew, which can be several thousand dollars per hour.
The fluid in the fluid-filled streamer also creates a number of problems. As the fluid-filled streamer is towed, the tow vessel tugging on the streamer creates bulge waves within the fluid in each active stream section. Bulge waves are created within the streamer by low frequency excitation such as that caused by paravanes in rough seas. The bulge waves result from interactions between the internal mechanical members of the streamer, the fill fluid, and the pliant outer cable skin. Bulge waves tend to impart noise into the hydrophones, thereby degrading the quality of the detected signals. Additionally, the ripples at the water surface affect fluid-filled streamers. Each time a surface wave impinges upon the streamer cable, it creates an acoustic noise source, which transmits through the fluid in the form of noise. Additionally, kerosene typically used in fluid filled streamers is toxic and highly flammable, which creates safety, health and environmental problems. Moreover, streamer filler fluid leaking into the ocean is hazardous to marine life.
As noted earlier, prior art streamer cables utilize between 96 and 150 hydrophones per 100-meter section. Groups of hydrophones are usually used to detect signals corresponding to a single point in the cable. Signals from all the hydrophones in each group are combined to reduce the effect of various types of noises present in the fluid-filled streamer cables. The cable design of the present invention is inherently less noisy and, thus, allows for the use of relatively fewer numbers of hydrophones per active cable section.
Fluid-filled streamer cables are but one of numerous designs and configurations known in the art. Gel-filled and solid streamers are also employed. Each type of streamer, however, entails its own characteristics, advantages and disadvantages. Solid streamers are durable and substantially immune to bulge wave noise, however, solid streamers can also be somewhat stiff and unwieldy during deployment and storage. Solid streamers also tend to couple longitudinal forces induced by cable motion and couple the associated mechanical noise into a solid-streamer hydrophone. Unlike solid streamers, fluid-filled streamers do not transfer as much mechanical noise and are more pliable during deployment. Fluid-filled streamers, however, are less durable than solid streamers and are susceptible to noise created by bulge waves as discussed above.
Moreover, as discussed above, fluid-filled streamers typically are thin-skinned and easily penetrated. The fluid in the streamers is typically toxic and flammable, which presents environmental and safety hazards when the fluid leaks from a breach in the skin. Thus, filler fluid leaking from a fluid-filled streamer into the ocean or on to the deck of a towing vessel is problematic. Fluid leaks allow water into the electronic streamer components. Thus, electronics can be jeopardized when water leaks into a fluid-filled streamer. Moreover, fluid-filled streamers typically lose buoyancy and sink when invaded by water.
Gel-filled streamers provide more flexibility than solid streamers and are more durable than fluid-filled streamers. Gel-filled streamers do not present the environmental concerns associated with spills from fluid-filled streamers. Gel-filled streamers do not leak or lose buoyancy when ruptured. The gel remains in place in the streamer and is not displaced by water should a leak occur in the streamer. Typical gel-filled streamer hydrophones designs, however, are sensitive to noise caused by cable motion and turbulent flow on the exterior streamer surface. Thus, there is a need to isolate these hydrophones from mechanical forces originating at the surface of the streamer and inside the streamer body.